Adds scalafmt formatting to the fmt command for scala files.

Refactored ScalaFmt class into a base class, with two sub classes ScalaFmtCheckFormat (checks if files are formatted correctly) and ScalaFmtFormat (formats the files).  This ensures that the same version of scalafmt is used for both.

Both of these are currently turned off in pants.ini. Skip=True

Testing Done:
New Integration Test Case
CI passes [pantsbuild#3936](pantsbuild#3963)

Reviewed at https://rbcommons.com/s/twitter/r/4312/

As described in the [updated release docs](http://www.pantsbuild.org/release.html#preparation-for-the-release-from-the-stable-branch), this review adds the 1.2.0rc2 notes in master, which will then be cherry-picked and released from the `1.2.x` branch.

- Update the release notes for a 1.2.0rc2 release.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/168717837

Reviewed at https://rbcommons.com/s/twitter/r/4326/

- Add and use a target-types option to avoid formatting all targets.
- Fix output of suggested `./pants fmt` command.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/169065521

Bugs closed: 3957, 3990

Reviewed at https://rbcommons.com/s/twitter/r/4328/

Prepare the 1.2.0rc3 release. As with 1.2.0rc2, the actual release will go out from the `1.2.x` branch.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/169095919

Reviewed at https://rbcommons.com/s/twitter/r/4329/

Testing Done:
CI passed: https://travis-ci.org/pantsbuild/pants/builds/168784588

Reviewed at https://rbcommons.com/s/twitter/r/4322/

Previously the check was too restrictive and considered targets that did
not own files when the single publish ownership check is only intended
to prevent publishing the same file in more than one package.

Testing Done:
Tested this over in Aurora ad-hoc and it solved the publish problem
there as well as passing the new test emulating the Aurora `prep_command`
arrangement.

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/169610485

Bugs closed: 3968, 3969

Reviewed at https://rbcommons.com/s/twitter/r/4315/